added description of "--priority=rt"

programs/zstd.1

@@ -289,7 +289,8 @@ and weight typically 100x the target dictionary size (for example, 10 MB for a 1
 .TP
 .B \-B#
  cut file into independent blocks of size # (default: no block)
-
+.B \--priority=rt
+ set process priority to real-time
 
 .SH ADVANCED COMPRESSION OPTIONS
 .TP